Corrugated Architecture of the Okanagan Valley Shear Zone and the Shuswap Metamorphic Complex, Canadian Cordillera

Corrugated architecture of the Okanagan Valley shear zone and the Shuswap metamorphic complex, Canadian Cordillera

Sarah R. Brown1,2,*, Graham D.M. Andrews1,3, and H. Daniel Gibson2 1DEPARTMENT OF GEOLOGICAL SCIENCES, CALIFORNIA STATE UNIVERSITY–BAKERSFIELD, 9001 STOCKDALE HIGHWAY, BAKERSFIELD, CALIFORNIA 93311, USA 2DEPARTMENT OF EARTH SCIENCE, SIMON FRASER UNIVERSITY, 8888 UNIVERSITY DRIVE, BURNABY, BRITISH COLUMBIA V5A 1S6, CANADA 3DEPARTMENT OF GEOLOGY AND GEOGRAPHY, WEST VIRGINIA UNIVERSITY, 98 BEECHURST AVENUE, MORGANTOWN, WEST VIRGINIA 26506, USA

ABSTRACT

The distribution of tectonic superstructure across the Shuswap metamorphic complex of southern British Columbia is explained by east-west– trending corrugations of the Okanagan Valley shear zone detachment. Geological mapping along the southern Okanagan Valley shear zone has identified 100-m-scale to kilometer-scale corrugations parallel to the extension direction, where synformal troughs hosting upper-plate units are juxtaposed between antiformal ridges of crystalline lower-plate rocks. Analysis of available structural data and published geological maps of the Okanagan Valley shear zone confirms the presence of≤ 40-km-wavelength corrugations, which strongly influence the surface trace of the detachment system, forming spatially extensive salients and reentrants. The largest reentrant is a semicontinuous belt of late Paleozoic to Mesozoic upper-plate rocks that link stratigraphy on either side of the Shuswap metamorphic complex. Previously, these belts were considered by some to be autochthonous, implying minimal motion on the Okanagan Valley shear zone (≤12 km); conversely, our results suggest that they are allochthonous (with as much as 30–90 km displacement). Corrugations extend the Okanagan Valley shear zone much farther east than previously recognized and allow for hitherto separate gneiss domes and detachments to be reconstructed together to form a single, areally extensive Okanagan Valley shear zone across the Shuswap metamorphic complex. If this correlation is correct, the Okanagan Valley shear zone may have enveloped the entire Shuswap metamorphic complex as far east as the east-vergent Columbia River–Slocan Lake fault zones.

LITHOSPHERE; v. 8; no. 4; p. 412–421 | Published online online 21 June 2016 doi:10.1130/L524.1

INTRODUCTION the shear zone (Spencer and Reynolds, 1991). Some corrugations within core complexes in southwest Arizona can be followed for up to 40 km Ductile shear zones are rarely planar across large areas (Candela et al., parallel to the extension direction (Spencer and Reynolds, 1991). Late- 2009). Nonplanar geometry is manifested by linear features normal to the stage doming of the detachment surface causes corrugations to become strike of the mean plane and parallel to the direction of slip, e.g., slickenlines, doubly plunging and to produce a dome-and-basin structural topography mullions, and, at the largest scales, corrugations. Corrugations differ from (Fletcher et al., 1995). slickenlines and mullions in that they are large enough (with wavelengths of Corrugations are typically recognized by map-scale features (Fig. 1), hundreds of meters to tens of kilometers) to deform the entire shear zone and including: (1) sinuous detachment fault traces; (2) juxtaposed antiforms the rocks of the adjacent upper plate and lower plate. Corrugations are com- and synforms, which result in a convolute map trace of shear zone salients mon in extensional shear zones and metamorphic core complexes in both and reentrants, respectively; and (3) klippen of upper-plate rocks with continental and oceanic crust (Whitney et al., 2013), suggesting a genetic spoon-shaped geometries isolated on top of the surrounding lower plate link between the formation of core complexes and corrugated detachments (Chauvet and Sérrane, 1994; Frost et al., 1996) and elongated parallel to (Singleton, 2013). Corrugations have been recognized on detachments in the transport direction. The juxtaposition of alternating ridges of resis- the southwest United States (John, 1987; Spencer and Reynolds, 1991; tant lower-plate rocks with keels of recessive upper-plate rocks affects Davis et al., 1993; Mancktelow and Palvis, 1994; Frost et al., 1996; Fowler the topographic expression of the detachment surface so that structurally and Calzia, 1999; Singleton, 2013), Baja California, Mexico (Seiler et al., lower units are topographically higher. 2010, 2011), western Norway (Johnston and Hacker, 2005), the Aegean Sea Corrugations may form during the exhumation of core complexes, and (Wawrzenitz and Krohe, 1998), the Swiss Alps (Mancktelow and Palvis, some precede brittle deformation and cooling below the Curie temperature 1994), Papua New Guinea (Spencer, 2010; Daczko et al., 2011), central (Livaccari et al., 1995), whereas others form primarily in the brittle regime.

Sulawesi (Spencer, 2010), the Himalaya (Murphy and Copeland, 2005; The origins of corrugations (Singleton, 2013) include: (1) uniaxial (s1 =

Spencer, 2010), the Mid-Atlantic Ridge (Tucholke et al., 1998), and the s2 > s3; e.g., Fletcher and Bartley, 1994) and triaxial strain (s1 > s2 > s3; Philippine Sea (Harigane et al., 2008; Spencer and Ohara, 2013). Fig. 1; e.g., Fossen et al., 2013), resulting in horizontal shortening per- Corrugations are upright or steeply inclined, open, parallel folds of pendicular to the extension direction; (2) synemplacement warping of the detachment shear zones (Fig. 1), typically with wavelengths of 200 m surface by plutons or diapiric gneiss domes; and (3) rheological contrasts to 20 km and amplitudes of 30–2000 m. The fold axes of corrugations between upper- and lower-plate rocks. The importance of triaxial strain are characteristically parallel to the principal stretching lineation within regimes and high viscosity contrasts (~600:1) has been demonstrated in experiments, especially under transtension (e.g., Grujic and Mancktelow, *[email protected] 1995; Venkat-Ramani and Tikoff, 2002; Le Pourhiet et al., 2012).

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syn-extensional break-away fault deposits σ1 σ3 allochthonous hanging wall superstructure half-graben detachment fault antiformal mylonite zone corrugation autochthonous infrastructure σ2

synformal corrugation & re-entrant hanging wall half-graben

klippe PLAN VIEW upper plate

σ2

mylonite zone

lower plate

X detachment σ fault 1 σ3

Figure 1. Schematic model of a corrugated detachment and core complex, adapted from Fossen (2010). Note the presence of upper-plate rocks preserved on the corrugated detachment surface as klippen and reentrants, and the curvilinear surface trace of the detachment.

This paper describes synextensional corrugations of the Okanagan Val- belong to both the parautochthonous Kootenay terrane within the Omin- ley shear zone, which forms part of the western boundary of the Shuswap eca morphogeologic belt (Fig. 2, inset) and the accreted Quesnel terrane metamorphic complex in the southern Canadian Cordillera (Fig. 2). We within the Intermontane belt (Okulitch, 1979; Gabrielse et al., 1991). Some used field observations and analysis of geological maps to demonstrate Eocene rocks were deposited directly onto exhumed basement (Glombick how kilometer-scale corrugations influence the surface trace of the Okana- et al., 1999). Late, high-angle normal faults disrupt the Okanagan Val- gan Valley shear zone, and how they control the distributions of hanging- ley shear zone and locally juxtapose lower-plate rocks against the upper wall (upper-plate) and footwall (lower-plate) lithologies. We then applied plate, including at the margins of the horst-like Kettle–Grand Forks and this knowledge to examine competing models of the tectonostratigraphic Valhalla gneiss domes (Fig. 2). architecture of rocks in the upper plate of the Shuswap metamorphic Although the Okanagan Valley shear zone appears to be an important complex and to reconcile different estimates of crustal extension across bounding structure along which significant exhumation of the Shuswap the Okanagan Valley shear zone. metamorphic complex was accommodated, the magnitude of extension across the Okanagan Valley shear zone is widely debated (e.g., Whitney GEOLOGICAL SETTING et al., 2013). Estimates of the magnitude of extension across the shear zone vary from 0 to 90 km, without any along-strike correlation to latitude The Shuswap metamorphic complex is the largest metamorphic core (Brown et al., 2012, and references therein). In some areas, for example, complex in North America (Coney, 1980), and it underpins the southern between 49°N and 49°30′N, at Kelowna, and north of 51°45′N (Fig. 2), Canadian Cordillera in British Columbia and adjacent parts of Washington extension is estimated at 30–90 km based on shear zone geometry, the State (Fig. 2; Armstrong, 1982; Parrish et al., 1988). The western margin of depth from which lower-plate rocks were exhumed, and possible Eocene the Shuswap metamorphic complex was exhumed from midcrustal levels magmatic pinning points (e.g., Tempelman-Kluit and Parkinson, 1986; in the Eocene along a 450-km-long, west-dipping, low-angle detachment Bardoux, 1993; Johnson and Brown, 1996; Brown et al., 2012). In contrast, system (Johnson and Brown, 1996). Herein, we refer to the segment south at Vernon and Osoyoos (Fig. 2), apparent extension is much less, and it is of 51°N latitude as the Okanagan Valley shear zone, including a 1–2-km- less clear that the Mesozoic upper plate is allochthonous (Okulitch, 1987; thick distributed brittle to ductile shear zone (Fig. 1; Brown et al., 2012). Thompson and Unterschutz, 2004; Glombick et al., 2006b). Glombick The lower plate is composed of Proterozoic to Mesozoic high-grade et al. (2006b) estimated horizontal extension across the Okanagan Val- metamorphic rocks (sillimanite-bearing, amphibolite and granulite facies); ley shear zone at Vernon (50°15′N; Fig. 2) to be 0–12 km, based on the peak metamorphism occurred at ca. 98–92 Ma, followed by exhumation apparent lateral continuity of Paleozoic stratigraphy across the Shuswap during ca. 60–48 Ma (Brown et al., 2012). The upper plate consists of metamorphic complex (Thompson et al., 2006). This is important because (1) greenschist- to amphibolite-facies Paleozoic and Mesozoic marine the Paleozoic rocks straddling the Shuswap metamorphic complex at metasedimentary and metavolcanic rocks, and (2) nonmetamorphosed 50°15′N are critical to understanding the pre–Okanagan Valley shear Eocene terrestrial sedimentary and volcanic rocks deposited in supra- zone extent of Quesnellia and Kootenay terrane stratigraphy and the late detachment basins (McClaughry and Gaylord, 2005) and NNE-trending Paleozoic paleogeography and metallogeny of the southern Canadian grabens (Suydam and Gaylord, 1997) on the upper plate. Paleozoic rocks Cordillera (Paradis et al., 2006; Thompson et al., 2006; Lemieux et al.,

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51N 2007; Kraft, 2013). Furthermore, attempts to correlate lithostratigraphy within the Kootenay arc east of the Shuswap metamorphic complex and 050 similar successions west of the Shuswap metamorphic complex (Fig. 2, km OVsz inset) are hindered by large-magnitude extension across the metamorphic complex, and especially along the Okanagan Valley shear zone. Therefore, T-O the conclusion that the Okanagan Valley shear zone is absent or insignificant at Vernon (Glombick et al., 2006b) becomes a key observation. How can belts of non- or weakly extended, apparently autochthonous, upper-plate rocks be reconciled with immediately adjacent, greatly extended metamorphic basement? Glombick et al. (2006b) reconciled Vernon Eocene exhumation ages within the Shuswap metamorphic complex and the absence of a significant bounding shear zone (i.e., Okanagan Valley shear zone) by proposing a model of eastward ductile flow of a mid-

50N crustal channel from the Late Cretaceous through to the early Eocene (see Ok also Brown and Gibson, 2006; Gervais and Brown, 2011). However, this Kelowna F interpretation still requires unique local conditions where the Okanagan Valley shear zone must be absent or insignificant at 50°15′N, but present OVsz and significant (>30 km extension) to the north and south (Brown et al., OkM 2012). It is unlikely that even a nascent midcrustal channel flow would S have been restricted to the Vernon area only.

OUTCROP- TO MAP-SCALE ANALYSIS OF CORRUGATIONS ALONG THE OKANAGAN VALLEY SHEAR ZONE

Evidence for corrugations includes outcrop-scale folds, the sinuous Fig. 3 trace of the Okanagan Valley shear zone, and the map patterns of upper- G K plate (supradetachment basins and outliers) and lower-plate (inliers) lithol- 49N Osoyoos ogies in the southern Okanagan valley (Fig. 3; “grooves” of Tempelman- Kluit and Parkinson, 1986; Tempelman-Kluit, 1989). Small-scale, open, upright, parallel folds, tens to hundreds of meters in TC RG wavelength and amplitude, are observed in outcrop within the Okanagan gneiss (Figs. 4A–4C), the principal lithological unit within the southern Okanagan Valley shear zone (Brown et al., 2012). The axial traces of N OVszOkD these folds consistently trend toward 290°, parallel to the local stretching lineation (Fig. 3); poles to foliation measured across corrugations form symmetrical girdle distributions with interlimb angles greater than 120° RG (Fig. 3, inset). Smaller-scale folds are associated with thinner mechanical layering, such that 100-m-amplitude folds of the Okanagan gneiss contain 120W 119W 118W parasitic 10-m- and 1-m-scale folds of the gneissic foliation (Figs. 4A and 4B). Upright folds refold earlier intrafolial folds within the Okanagan Eocene sedimentary BC & volcanic rocks upper Canada gneiss (Fig. 4C), but they are not associated with a penetrative axial plane Quesnel terrane Intermontane Belt plate Omineca Foreland Belt N cleavage. We interpret these upright, open, parallel folds to be displace- Kootenay terrane ment-parallel corrugations of the Okanagan Valley shear zone. (parautochthonous)

lower plat The 100-m- to kilometer-scale folds of the Okanagan Valley shear

Shuswap metamorphic complex B elt zone are exhibited in local- and regional-scale geological maps as highly undiff. metamorphic rocks AB sinuous lithological contacts, foliations, and traces of the Okanagan Val- Vancouver V Paleoproterozoic - Paleozoic North SMC American basement and cover ley shear zone (Figs. 1 and 3; Tempelman-Kluit and Parkinson, 1986; sequence e Tempelman-Kluit, 1989). The fault trace is strongly curvilinear imme- plutons normal fault Seattle diately south and east of Okanagan Falls (OF; Fig. 3), where it bounds USA WA a prominent southeast-closing reentrant of Eocene rocks (Dusty Mac Okanagan Valley shear zone ID mine); we contend that this pattern is best explained as the result of Figure 2. Regional geology adapted from Johnson (2006), Kruckenberg the intersection between topography and a kilometer-scale synformal et al. (2008), Brown et al. (2012), and Kraft (2013); F—Fauquier, G— corrugation within the Okanagan Valley shear zone. A corresponding Greenwood, K—Kettle–Grand Forks Dome, Ok—Okanagan complex, northwest-closing salient of Okanagan gneiss is likewise interpreted to OkD—Okanogan Dome, OkM—Okanagan Mountain, OVsz—Okanagan reflect an antiformal corrugation. A total of ~500 m of structural relief is Valley shear zone, RG—Republic graben, TC—Toroda Creek graben, accommodated across each of the largest corrugations. T-O—Thor-Odin Dome, S—Summerland. Inset: Simplified map of the The Okanagan Falls reentrant can be extrapolated to the southeast southern Canadian Cordillera showing morphogeological belts and the outline of the Shuswap metamorphic complex (SMC), the Valhalla to the Venner Meadows klippen and west further into the White Lake gneiss dome (V), and the footprint of the main map shown in Figure Basin (Fig. 3), both of which are composed of similar Eocene rocks. 3. BC—British Columbia; AB—Alberta; ID—Idaho; WA—Washington. Synformal structure is evident from mapping and exploratory drilling; at

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Okanagan Lake N masl AA0 OVsz ’

1500 150 11 V Q:- Quaternary 1000 Skaha Lake N 500 500 Egn500 V V 43 V V -500 V V V 2 km Penticton V Evs:- undifferentiated Eocene 49° 30’

8 upper 17 sedimentary & volcanic rocks plate V 7 Q 21 1 V V 21 16 uTrs:- Shoemaker Fm V MzEgd 15 8 1 V V 1 21 V MzEgd:- Okanagan batholith V 1000 V 50 Ec 21 14 V Skaha V V Lake Evs V V 24 76 30 55 5 13 V V 2 1500 V V 49 Kbs OVsz V V 25 2 V 19 V V 19 Ec:- Coryell syenite V 5 A V Ec V V 43 Egn:- Okanagan gneiss V V 30 V V 1 Evs 10 gradation between V V 40 V V V 4 Egn & MzEgd V V 45 10 V V V Egn lower V V 20 30 plate X 87 4 V V Dusty Mac Kbs:- diorite pluton V V 4 25 20 OF V 5 V V 50 6 9 V V 2 24 V White Lake 38 MzEgd:- Okanagan batholith 50 19 20 Basin 8 12 V V V 15 20 V V V 7 7 18 V 34 N V X V 20 X A’ V V V 2 V 11 1 3 V 40 V V 74 V bedding dip foliation dip V V V V 49° 18 ’ 6 30 5 V 30 V V 6 45 Evs V V 2 6 Venner 7 40 6 1 V 9 V Meadows elongation lineation 6 fold hinge uTrs 3 V V 6 10 1 1 V 3 1 16 V 16 10 1 detachment (observed, interpreted, inferred)

6 2 1 00 1000 40 Egn 4 1500 34 2 corrugation fold axial traces

500 Q MzEgd X synformal MzEgd 50 0 km 5 0 antiformal 119° 30’ W 119° 18’ W cylindrical best lower hemisphere N fit of foliations N N equal area 046/85/SE e1 = 06/311

e3 = 05/316 poles to foliation top down to 295 top down to 295

fold axes lineations e1 = 01/115

Figure 3. Geological map and cross section of the southern Okanagan Valley shear zone (OVsz) modified from Church (1973) and Brown et al. (2012). Below: Stereonet projections of linear (stretching lineations [maximum eigenvector 01/115] and fold hinges [06/311]) and planar data (poles to foliation, Kamb contoured [minimum eigenvector 05/316]) from the Okanagan gneiss; masl—meters above sea level; OF—Okanagan Falls.

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ABlooking east looking southeast

Figure 4. (A) Interpreted field photo of a major corrugation fold pair in the Okanagan gneiss southeast of Okanagan Falls, looking east. Fore- ground is ~1 km across. (B) Oblique low-angle view of 1-m-scale corrugations in the Okanagan gneiss. (C) Cross section through a set WNW of upright corrugation folds that refold earlier recumbent intrafolial C looking south D looking north folds within the Okanagan gneiss. (D) View north of high-angle normal faults (throw direction indi-

X X cated) cutting the foliation of the

Okanagan gneiss, McIntyre Bluff U (Universal Transverse Mercator 11N 316524 m E, 5462379 m N). Cliff is 170 m high.

1 m

Dusty Mac and Venner Meadows, drilling penetrated the Okanagan Val- portion of the Okanagan Valley shear zone, the Okanogan Dome (Fig. 5) ley shear zone and the underlying Okanagan gneiss (Morin, 1989; Evans, is a large salient and corrugated gneiss dome where doubly plunging sub- 1990). The White Lake Basin is mapped as a supradetachment basin on domes of paragneiss alternate with synforms of structurally higher ortho- top of the Okanagan Valley shear zone (Church, 1985; McClaughry and gneiss (Kruckenberg et al., 2008). The Okanogan Dome is truncated to the Gaylord, 2005; Brown et al., 2012), and we infer that these locations can east by the N-trending Eocene Republic graben and is heavily fractured be reconstructed as a single, WNW-trending, synformal corrugation, the by the same array of N- and NE-trending fractures and normal faults as White Lake–Venner Meadows reentrant (WLB-VM; Fig. 5). The White the Okanagan gneiss. WNW-trending corrugations are also described in Lake–Venner Meadows reentrant is bounded to the north by a prominent the far northern Okanagan Valley shear zone (Johnson, 1994) and within west-closing salient that exposes a ca. 104 Ma hornblende-diorite pluton the Thor-Odin and Kettle–Grand Forks gneiss domes (Fig. 2; Cubley of the lower plate (Kbs; Fig. 3) as an inlier exposed through the Okanagan and Pattison, 2012). gneiss (Brown et al., 2012). Weakly developed foliation within the diorite At the regional scale, the Okanagan Mountain and Okanogan Dome is antiformal and parallel to the adjacent covering gneiss. salients are separated by a laterally extensive belt of semicontinuous Paleo- Prominent arrays of high-angle (dipping 50°–80°), N-, NNE-, and NE- zoic marine metasedimentary and metavolcanic upper-plate rocks of the trending fractures and normal faults are superimposed on the corrugated Osoyoos-Greenwood reentrant between Osoyoos and Greenwood, Brit- Okanagan gneiss and its plutonic inliers (Figs. 4D and 5; Ross, 1974; ish Columbia, at ~49°N (Fig. 5; Okulitch, 1987; Massey, 2006; Massey Eyal et al., 2006; Brown et al., 2012). The apparent extension direction and Duffy, 2008). It is partly buried by the Eocene Toroda Creek graben responsible for the formation of these high-angle fractures and faults and associated sedimentary and volcanic rocks (Suydam and Gaylord, parallels the net displacement vector for the Okanagan Valley shear zone 1997), and it is truncated to the east by the Kettle–Grand Forks gneiss (toward the WNW). These are inferred to be late-stage brittle structures dome. The reentrant extends 20 km west of Osoyoos across the previously developed during and after exhumation of the lower plate of the Okanagan mapped Okanagan Valley shear zone in the Okanagan Valley north to Valley shear zone as it approached the surface, and they may be related Keremeos (Fig. 5; Okulitch, 1973). The Paleozoic succession is composed to faults that bound the overlying supradetachment basins. of marine metasedimentary and metavolcanic rocks of the Kobau and Analysis of regional-scale geological maps of immediately adjacent Anarchist Groups (Okulitch, 1973), and the ophiolitic Knob Hill complex sections of the Okanagan Valley shear zone reveals a similar pattern of (Massey, 2006); all have uncertain affinity and are variably assigned to the corrugations. To the north, Okanagan Mountain (Fig. 2) is a prominent, allochthonous Okanagan or Quesnel terranes. The Knob Hill complex is heavily fractured salient where a lower-plate pluton is exposed through repeated in a series of pre-Jurassic, north-dipping, south-vergent thrusts the Okanagan gneiss. The Okanagan Mountain salient (Fig. 5) is bounded sheets south of Greenwood (“G” in Fig. 5; Fyles, 1990; Massey, 2006), north and south by reentrants dominated by Eocene supradetachment forming a broad, open synform about a NNW-trending axis. Lineations basins at Kelowna (Bardoux, 1993; Bardoux and Mareschal, 1994; Oku- on shear surfaces within and between the thrust sheets trend ENE–WNW litch, 2013) and Summerland, respectively. To the south, within the U.S. (i.e., broadly parallel to the present strike of the thrust planes); the shear

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Chase 18W sense is not recorded (Fyles, 1990). Most thrusts in this part of the Cana-

W X

0 antiform X dian Cordillera typically strike SSE and dip westward, corresponding

2 1 to repeated east-directed shortening throughout the late Paleozoic and Mesozoic as Panthalassan terranes accreted to Laurentia (Dickinson, 2004; Colpron et al., 2007). We interpret the north-dipping Greenwood thrusts as Vernon Oyama Lake having been folded during Eocene extension about an ESE–WNW axis, salient Ve parallel to the original pre-Jurassic shortening direction. rnon - Fauquier We interpret this belt to be a large-scale reentrant of the Okanagan re-entran Pinnacles Valley shear zone hanging wall, based on (1) the broad synformal nature 51N Thrust of the Paleozoic lithostratigraphy; (2) its presence between lower-plate Kelowna t re-entrant Fauquier highs dominated by gneiss-cored salients; and (3) the presence of north- dipping, Upper Paleozoic to Triassic thrusts. If this interpretation is cor- Kelowna rect, some of the lithostratigraphic complexity within the reentrant may be due to hitherto unrecognized dismemberment during stretching of the Okanagan Mountain salient upper plate (e.g., Fig. 1). S A second, larger belt of Paleozoic and Mesozoic marine metasedi- Summerland re-entrant mentary and metavolcanic rocks juxtaposed against high-grade basement straddles the Shuswap metamorphic complex between Vernon and White Lake Fauquier (Fig. 5) at ~50°15′N: the Vernon-Fauquier reentrant. The belt Basin is ~40 km wide and over 150 km long, and it extends across both the X WLB - VM Okanagan Valley shear zone and Shuswap metamorphic complex, where re-entrant X it is disrupted by numerous NNE-trending normal faults. It is a broad, WNW-trending, synformal structure (Carr, 1995) that includes 10-km-

Keremeos Granby Fault scale, upright antiforms and synforms (e.g., Chase antiform—Okulitch, 1984; Vernon antiform—Glombick et al., 2006a) and the south-dipping, G Osoyoos - top-to-the-N Pinnacles thrust (Fig. 5). Okulitch (1984) interpreted the Greenwood southern margin of the belt as a compressional shear zone within the 49 N Osoyoos re-entrant Kettle River Faul underlying Shuswap metamorphic complex, but everywhere the contacts TC are ambiguous because of complex small-scale deformation, metamor- Mount Hull salient K phism, and often very poor exposure. We interpret this belt to be a Shuswap metamorphic complex–spanning Burge Mtn. salient t reentrant, based on (1) the broad, WNW-trending (i.e., parallel to extension across the Okanagan Valley shear zone) synformal geometry; (2) the presence of kilometer-scale upright parasitic folds; and (3) the presence Stowe Mtn. of mutually divergent compressional shear zones. If this interpretation is salient correct, many, if not all, of the high-angle normal faults that disrupt the OkD RG reentrant may root into the underlying detachment surface. N The Upper Devonian Silver Creek Formation can be traced continu- ously through the reentrant and across the Shuswap metamorphic complex, providing a definitive stratigraphic correlation between the Kootenay arc 25 km to the east and the Eagle Bay assemblage to the west on either side of the Shuswap metamorphic complex (Kraft, 2013), and across the southern Eocene sedimentary and volcanic rocks Omineca belt (Fig. 2, inset; Fig. 5). This correlation cannot be made other Upper plate Paleozoic & Mesozoic rocks than in the reentrant and was used by Glombick et al. (1999, 2006b) to Shuswap metamorphic complex (lower plate) support the interpretation that the Okanagan Valley shear zone is absent or insignificant at Vernon; however, this interpretation contrasts with the plutonic intrusions many studies of the Okanagan Valley shear zone to the north and south of 50°15′N, where estimates of significant extension (~30–90 km) have OVsz OVsz inferred normal fault thrust fault been made (e.g., Tempelman-Kluit and Parkinson, 1986; Bardoux, 1993; Johnson and Brown, 1996; Brown et al., 2012). The identification of fracture and minor fault traces foliation traces synextensional, displacement-parallel corrugations within the Shuswap metamorphic complex allows for this to not be a zero-sum problem. If X mapped synform mapped antiform our corrugation model is correct, then the Okanagan Valley shear zone passes under the Vernon-Fauquier reentrant and is not exposed at Vernon synformal corrugation axis antiformal corrugation axis other than where it bounds the Aberdeen Gneiss (Kalamalka shear zone; Glombick et al., 2006a) around the Oyama Lake salient (Fig. 5). Instead, Figure 5. Schematic geological map of the distribution of upper-plate and the surface trace of the Okanagan Valley shear zone is the margin of the lower-plate rocks in the Shuswap metamorphic complex, corrugations, and our new interpretation of the trace of the Okanagan Valley shear zone. Vernon-Fauquier reentrant. Abbreviations are as in Figure 2; WLB-VM—White Lake Basin–Venner Mead- Combined, these observations suggest the presence of WNW-trending ows. Figure is adapted from Okulitch (1987) and Kruckenberg et al. (2008). corrugations all along the Okanagan Valley shear zone and the southern

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A discrete gneiss domes and detachments ~200 km W OVsz GF KF VSZ SLF E

Valhalla T K - GF gneiss Ok gneiss dome dome gneiss ? ?

B areally extensive, corrugated OVsz W OVsz GF KF VSZ SLF E Osoyoos - Greenwood re-entrant Vernon - Fauquier re-entrant Valhalla T K - GF gneiss gneiss ? Ok dome gneiss dome ? inferred inferred OVsz OVsz

Eocene graben-ﬁll parautochthonous upper plate

Figure 6. Simplified composite cross sections (W–E) showing alternative models of the architecture between 49°N and 50.5°N, compiled from observations throughout the Shuswap metamorphic complex. Major gneiss domains are in italics: Ok—Okana- gan, K-GF—Kettle–Grand Forks. Major faults and shear zones include: OVsz—Okanagan Valley shear zone, GF—Granby fault, KF—Kettle River fault, VSZ—Valkyr shear zone, SLF—Slocan Lake fault. T—Toroda Creek graben. (A) Depiction of the current understanding of gneiss domes and their bounding detachments and high-angle normal faults (e.g., Carr et al., 1987; Parrish et al., 1988; Simony and Carr, 1997; Cubley and Pattison, 2012). Note that the Valkyr shear zone is an E-throwing detachment, even though it dips to the west (Carr et al., 1987), because it is interpreted to be arched over the Valhalla dome and offset by the E-dipping Slocan Lake fault (see text for details). Also note that the downdip extensions of the Slocan Lake fault and Okanagan Valley shear zone have been identified at depth by seismic surveys but that the Granby and Kettle River faults, the Valkyr shear zone, and the faults bounding the Toroda Creek graben have not (Carr, 1995). (B) Depiction of how upper- plate reentrants (corrugations) potentially link the Okanagan Valley shear zone with the Kettle–Grand Forks and Valhalla gneiss domes. In this scenario, the Granby fault is a brittle W-directed overprint on the Okanagan Valley shear zone at the eastern termination of the Osoyoos-Greenwood reentrant. The Valkyr shear zone marks the eastern limit of the Okanagan Valley shear zone at the eastern termination of the Vernon-Fauquier reentrant. The alternative model proposed agrees with the Lithoprobe seismic data (Carr, 1995), where the Okanagan Valley shear zone is observed to be near horizontal between high-angle normal faults in the upper crust.

and central Shuswap metamorphic complex, and they allow constraint of lithologies (48–42 Ma—Ross, 1974; ca. 48–46 Ma—Parrish et al., 1988; the maximum wavelength to ~40 km and the maximum amplitude to 2–3 Bardoux, 1993; 54–49 Ma—Holder et al., 1990; ca. 48 Ma—Adams et al., km based on the present relief and inferred thickness of the Okanagan 2005). Although invoked often, no geodynamic model explains a phase Valley shear zone. of early Eocene, N–S–directed shortening independent of top-to-the-W extension along the Okanagan Valley shear zone. Moreover, there is no DISCUSSION evidence of an Eocene (i.e., late) N–S shortening event elsewhere in the southern Canadian Cordillera or adjacent parts of Washington State; the Meter- to kilometer-scale elongated antiforms and synforms within E–W–trending Yakima fold-and-thrust belt in southern Washington State is the Okanagan Valley shear zone and adjacent Shuswap metamorphic a Neogene structure (Campbell, 1989). Postdetachment buckling has been complex have hitherto been interpreted as the result of a late-stage, N–S documented in the Himalayan South Tibetan detachment (e.g., Godin et compressional folding event (e.g., “Phase 2”—Preto, 1970; “Phase 4”— al., 2006; Kellett and Grujic, 2012), but there shortening is perpendicular Ryan, 1973; “Phase 5”—Ross, 1975, 1981; Ross and Christie, 1979; to the detachment displacement direction (i.e., the buckle folds are parallel “D4”—Cubley and Pattison, 2012). However, the N-S–directed shorten- to the strike of the detachment); in the Okanagan Valley shear zone, the ing must be synextensional and of early Eocene age, because it folded corrugations are parallel to displacement. the then-active Okanagan Valley shear zone (53–48 Ma; Brown et al., We infer from the presence of laterally extensive, WNW-trending, 2012), and the folds themselves are offset by nonfolded, high-angle, brittle upright synforms and antiforms that the Okanagan Valley shear zone normal faults (Figs. 4D and 5; e.g., Eyal et al., 2006), similar to those and the adjacent upper- and lower-plate lithologies are corrugated (Fig. bounding the Toroda Creek (51–48 Ma; Suydam and Gaylord, 1997) and 5). The wavelengths and amplitudes of corrugations are controlled by Republic grabens (54–47 Ma; Holder et al., 1990), and the Kettle–Grand the thicknesses of layering in the host lithologies (i.e., buckle folds) and Forks gneiss dome (ca. 51 Ma; Cubley and Pattison, 2012). The upright are observed from 1 m scale to tens-of-kilometers scale. These corruga- folds are crosscut by nonfolded N- and NNE-trending, alkaline dikes that tions are identified throughout the Okanagan Valley shear zone (e.g., intrude the Okanagan Valley shear zone, and both upper- and lower-plate Okanagan Mountain, Okanogan Dome; Fig. 2), and the adjacent Shuswap

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metamorphic complex (e.g., Kettle–Grand Forks and Valhalla gneiss ages (59–50 Ma), and (2) has rapid Eocene exhumation ages (55–51 Ma). domes; Fig. 2). The geologic relationships and limited range of ages (ca. However, the Granby fault is interpreted to be largely brittle in nature and 54–48 Ma) imply strongly that the corrugations formed during synexten- only responsible for the latest, low-temperature exhumation of the Grand sion and as part of a continuous and evolving process of extension along Forks complex. The mechanism for its more significant Eocene (52–50 a low-angle, ductile detachment (the Okanagan Valley shear zone) that Ma) high-temperature decompression of at least 2.5 kbar remains elusive evolved to be dissected by high-angle, brittle normal faults. The most (Cubley and Pattison, 2012; Cubley et al., 2013), but our model suggests plausible explanation for the formation of corrugations is, therefore, hori- that the Okanagan Valley shear zone may have been the exhuming structure

zontal shortening perpendicular to extension (i.e., in the s2 direction) along that was progressively overprinted by the W-directed brittle deformation of the contemporaneous Okanagan Valley ductile shear zone detachment the Granby fault. On the basis of these data, we propose that the Granby (Fig. 2; e.g., Singleton, 2013). As the Shuswap metamorphic complex fault is the easternmost brittle expression of the Okanagan Valley shear was gradually exhumed along the Okanagan Valley shear zone, it passed zone along the margin of the Osoyoos-Greenwood reentrant (Fig. 6B). through the brittle-ductile transition diachronously (50–48 Ma), allow- The Vernon-Fauquier reentrant extends as far east (~200 km) as the ing for brittle fractures and faults to progressively overprint slightly older western margin of the Valhalla gneiss dome (Fig. 2, inset; Carr et al., ductile shear fabrics and corrugations (e.g., John, 1987; Frost et al., 1996; 1987; Simony and Carr, 1997), where the kinematics of extension reverse, Eyal et al., 2006). The southern Canadian Cordillera experienced the onset accommodated by broadly coeval top-to-the-E shear zones that include the of transtensional deformation during the early Eocene (ca. 52 Ma; Price, Valkyr shear zone and the Columbia River fault farther north (e.g., Parrish 1979; Price et al., 1981; Price and Carmichael, 1986; Harms and Price, et al., 1988; Johnson and Brown, 1996). Thus, we tentatively suggest that 1992; Struik, 1993; Andronicus et al., 2003) following plate reorganiza- the Valkyr shear zone marks the eastern termination of the Okanagan Val- tion at the North America–Farallon margin (Lonsdale, 1988; Monger and ley shear zone along the eastern margin of the Vernon-Fauquier reentrant Price, 2002). Activity on the Okanagan Valley shear zone (ca. 54–48 Ma) (Fig. 6B). If this is correct, activity on the Okanagan Valley shear zone was contemporaneous with the phase of transtensional deformation and was probably coeval with, or only slightly older than, the Valkyr–Colum- supports other studies where corrugation formation has been associated bia River fault system, and the maximum total displacement as measured with transtensional strain (e.g., Bartley et al., 1990; Chauvet and Sérrane, along the axes of antiformal corrugations would be up to 150 km. 1994; Venkat-Ramani and Tikoff, 2002). The presence of discrete gneiss domes within the Shuswap metamorphic complex is explained by their position in the footwalls of E-throwing, Is the Shuswap Metamorphic Complex Bounded by a Single high-angle normal faults (Kettle River fault, ca. 51 Ma—Cubley and Pat- Disrupted Detachment? tison, 2012; Slocan Lake fault, ca. 54–47 Ma—Carr et al., 1987). These faults have similar geometry to those disrupting the Okanagan Valley shear The Shuswap metamorphic complex is distinctive for its size and in zone in the Okanagan Valley (Eyal et al., 2006; Brown et al., 2012). In having several discrete gneiss domes (Fig. 2), but there are few studies that both cases, the amount of vertical motion on brittle high-angle faults is have attempted to integrate them in a single model. Parrish et al. (1988) poorly constrained and probably ≤3 km (e.g., Cubley and Pattison, 2012); explained each gneiss dome as a Proterozoic basement–cored nappe thrust however, this is more than sufficient to disrupt the upper plate, which eastward in the Cretaceous–Paleocene and then reversed and extended would otherwise cover the gneiss domes (cf. Carr, 1995). The high-angle in the Eocene. This is, however, inconsistent with the following: (1) The normal faults likely also explain the relatively steep, 30°–35° westerly only deep-rooted shear zones imaged by Lithoprobe seismic experiments dips of the Valkyr shear zone and the Granby fault by footwall uplift and are the Shuswap metamorphic complex–bounding Okanagan Valley shear horizontal axis rotation. The progressive cutting off of earlier low-angle zone and Slocan Lake fault (Carr, 1995), and (2) there is no evidence for ductile shear zones by high-angle brittle normal faults (e.g., the Slocan Proterozoic basement coring the Okanagan, Kettle–Grand Forks, and/or Lake fault) and the development of horsts and grabens that dismembered Valhalla gneiss domes (e.g., Brown et al., 2012). Models of midcrustal the primary detachment surface and its lower- and upper-plate architec- channel flow (e.g., Brown and Gibson, 2006; Gervais and Brown, 2011) ture are characteristic of the later stages of large-scale exhumation of a adopt the Okanagan Valley shear zone as the upper margin of the channel buoyant core complex (e.g., Wernicke and Axen, 1988). The presence of throughout the Cretaceous–Paleocene midcrustal flow phase, satisfying discrete gneiss domes in the Shuswap metamorphic complex does not, the Lithoprobe data; however, they do not attempt to explain the presence therefore, preclude their formation as part of a single, much larger detach- of the other faults, shear zones, and gneiss domes. ment system (the Okanagan Valley shear zone), and instead likely reflects Our identification of corrugations and spatially extensive reentrants the effects of only the latest stages of crustal extension. extends the footprint of the Okanagan Valley shear zone much further east than previously thought (Fig. 5) and implies structural connections between CONCLUSIONS detachments and gneiss domes that have hitherto been considered as discrete (Fig. 6). For example, the Osoyoos-Greenwood reentrant extends Field observations and the interpretation of published geological maps eastward to the western margin of the Kettle–Grand Forks gneiss dome have identified corrugations that deform the Okanagan Valley shear zone and along the Granby fault (Preto, 1970; Parrish et al., 1988; Carr and Parkin- the juxtaposed upper and lower (Shuswap metamorphic complex) plates. son, 1989; Laberge and Pattison, 2007; Cubley et al., 2013), implying that Corrugations explain the undulating trace of the Okanagan Valley shear zone the Okanagan Valley shear zone and the Granby fault are part of the same and allow for structural linkages between klippen of similar-aged litholo- W-directed extensional system (Fig. 6B). The Okanagan Valley shear zone gies. Two 40-km-wide synforms form reentrants of upper-plate lithologies and the Granby fault share many common characteristics. The Granby fault that extend far across the Shuswap metamorphic complex. The recognition (1) is a top-down-to-the-W, low-angle detachment (Carr and Parkinson, of large-scale corrugations provides a permissible explanation for the dif- 1989; Laberge and Pattison, 2007; Cubley et al., 2013) that juxtaposes ferences determined for the magnitude of extension of the Okanagan Valley greenschist- to lower-amphibolite-facies Paleozoic and Mesozoic upper- shear zone, allows for models of Shuswap metamorphic complex–spanning plate rocks against sillimanite-grade, upper-amphibolite- to granulite-facies Paleozoic–Mesozoic stratigraphy to coexist with the presence of a major crystalline basement with Paleocene to early Eocene peak metamorphic Shuswap metamorphic complex–bounding detachment, and explains the

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relationship between the Okanagan and Kettle–Grand Forks gneiss domes, Davis, G.A., Fowler, T.K., Bishop, K.M., Brudos, T.C., Friedmann, S.J., Burbank, D.W., Parke, M.A., and Burchfield, B.C., 1993, Pluton pinning of an active Miocene detachment fault and potentially with the Valhalla gneiss dome also. These results bring system, eastern Mojave Desert, California: Geology, v. 21, p. 627–630, doi:10 .1130 /0091 models of Shuswap metamorphic complex extension via collapse of a large -7613(1993)021<0627:PPOAAM>2.3.CO;2. stack of basement-cored thrusts into doubt, but they do not preclude models Dickinson, W.R., 2004, Evolution of the North American cordillera: Annual Review of Earth and Planetary Sciences, v. 32, p. 13–45, doi:10.1146/annurev.earth.32.101802.120257. of midcrustal channel flow or typical core complex formation. Evans, G., 1990, Assessment Report for Diamond Drilling on the Dusty Mac Property, Osoyoos Mining District, B.C., N.T.S. 82E/5 Latitude 49°20′N, Longitude 119°32W: British Columbia ACKNOWLEDGMENTS Ministry of Energy, Mines and Petroleum Resources Report 20078, 209 p. Eyal, Y., Osadetz, K.G., and Feinstein, S., 2006, Evidence of reactivation of Eocene joint and We wish to thank Editor R. Damian Nance and reviewers John Singleton, Deta Gasser, and pre-Eocene foliation planes in the Okanagan core complex, British Columbia, Canada: Dawn Kellett. This study was funded by awards to Brown from Simon Fraser University, the Journal of Structural Geology, v. 28, p. 2109–2120, doi:10 .1016 /j .jsg.2006.06.004. American Association of Petroleum Geologists, and Sigma Xi; to Andrews from National Sci- Fletcher, J.M., and Bartley, J.M., 1994, Constrictional strain in a non-coaxial shear zone: Impli- ence Foundation HRD award 1137774, and to Gibson from a Natural Sciences and Engineering cations for fold and rock fabric development, central Mojave metamorphic core complex, Research Council of Canada (NSERC) Discovery grant. Jim Morin, Andrew Okulitch, and Brad California: Journal of Structural Geology, v. 16, no. 4, p. 555–570. Johnson provided helpful ideas and discussions during this study. Fletcher, J.M., Bartley, J.M., Martin, M.W., Glazner, A.F., and Walker, J.D., 1995, Large-magnitude continental extension: An example from the central Mojave metamorphic core REFERENCES CITED complex: Geological Society of America Bulletin, v. 107, p. 1468–1483, doi:10 .1130 /0016 -7606(1995)107<1468:LMCEAE>2.3.CO;2. 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